JP2746029B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine.

[0002]

2. Description of the Related Art In a diesel engine, in order to purify NOx, an engine exhaust passage is branched into a pair of exhaust branch passages, and a switching valve is disposed at a branch portion of these exhaust branch passages. 2. Description of the Related Art A diesel engine is known in which a catalyst is introduced alternately into one of the exhaust branch passages and a catalyst capable of oxidizing and absorbing NOx is disposed in each of the exhaust branch passages (see JP-A-62-106826). In this diesel engine, NOx in the exhaust gas guided into one exhaust branch passage is oxidized and absorbed by a catalyst disposed in the exhaust branch passage. During this time, the flow of exhaust gas into the other exhaust branch passage is stopped, and a gaseous reducing agent is supplied into the exhaust branch passage, and the reducing agent accumulates in the catalyst disposed in the exhaust branch passage. NOx that has been reduced is reduced. Then, after a while, the switching action of the switching valve stops the introduction of the exhaust gas into the exhaust branch passage from which the exhaust gas has been guided, and exhausts the exhaust branch passage into which the introduction of the exhaust gas has been stopped. Gas introduction is resumed.

[0003]

In order to promote the reduction of NOx in such a catalyst, it is preferable to increase the temperature of the catalyst. However, in the above-described diesel engine, the temperature of the catalyst does not increase so much. Therefore, there is a problem that the reduction of NOx cannot be sufficiently promoted.

[0004]

According to the present invention, to solve the above-mentioned problems, NOx is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and the oxygen concentration in the inflowing exhaust gas is reduced. NO that releases absorbed NOx when lowered
x absorbent is disposed in the engine exhaust passage, and heating means for heating the NOx absorbent is provided.
When the concentration of oxygen flowing into the NOx absorbent is decreased to release Ox, the concentration of oxygen in the exhaust gas coming into contact with the NOx absorbent falls below a predetermined concentration, and then the temperature is raised by the heating action of the heating means. The heating operation of the heating means is controlled so that the temperature of the NOx absorbent exceeds a predetermined temperature.

[0005]

When the NOx is to be released from the NOx absorbent in order to promote the action of releasing the NOx from the NOx absorbent, the temperature of the NOx absorbent is increased by the heating means. In this case, if the temperature of the NOx absorbent is increased when the oxygen concentration in the exhaust gas is high, the sulfur oxide SOx absorbed by the NOx absorbent together with NOx is stabilized in the NOx absorbent,
Even if the oxygen concentration in the exhaust gas is reduced to release Ox, SOx is kept in the NOx absorbent. Therefore, when SOx is not stabilized in the NOx absorbent and the oxygen concentration in the exhaust gas is reduced to release NOx, the exhaust gas which comes into contact with the NOx absorbent so that SOx is released from the NOx absorbent together with NOx. The heating operation of the NOx absorbent by the heating means is controlled so that the temperature of the NOx absorbent rises after the oxygen concentration in the gas falls below a predetermined concentration.

[0006]

FIG. 1 shows a case where the present invention is applied to a diesel engine. Referring to FIG. 1, reference numeral 1 denotes an engine body, 2
Represents a piston, 3 represents a combustion chamber, 4 represents a fuel injection valve, 5 represents an intake valve, 6 represents an intake port, 7 represents an exhaust valve, and 8 represents an exhaust port. The intake port 6 is connected to the air cleaner 11 via the corresponding branch pipe 9 and surge tank 10. On the other hand, the exhaust port 8 is connected to the exhaust manifold 12 and the exhaust pipe 1.
The exhaust pipe 14 includes a first exhaust passage 16 a and a second exhaust passage 16 b branched at a branch portion 15. The branch portion 15 is provided with a flow path switching valve 17 for causing exhaust gas discharged from the engine to flow mainly into one of the first exhaust passage 16a and the second exhaust passage 16b. The flow path control valve 17 is controlled by an actuator 18.

In the first exhaust passage 16a, a first N with heater is provided.
A casing 20a containing an Ox absorbent 19a is arranged, and a casing 20b containing a second heater-equipped NOx absorbent 19b is arranged in the second exhaust passage 16b. The first exhaust passage 16a and the second exhaust passage 16b
Downstream of the x absorbent 19a and the second NOx absorbent 19b, they are merged with each other. First NOx absorbent 19
a first reducing agent supply valve 2 in the first exhaust passage 16a
1a is arranged, and a second reducing agent supply valve 21b is arranged in the second exhaust passage 16b upstream of the second NOx absorbent 19b. The first reducing agent supply valve 21a and the second reducing agent supply valve 21b are connected to the reducing agent tank 2 via a corresponding first control valve 22a, second control valve 22b and supply pump 23, respectively.
4. The reducing agent tank 24 is filled with hydrocarbons such as gasoline, isooctane, hexane, heptane, light oil and kerosene, or hydrocarbons such as butane and propane which can be stored in a liquid state. Also, the first N
The first exhaust passage 16a downstream of the Ox absorbent 19a
An air-fuel ratio sensor 25a is disposed, and the second NOx absorbent 16
b, the second air-fuel ratio sensor 2
5b are arranged.

FIG. 2 shows the NOx absorbents 19a and 19 with heaters.
FIG. 2 shows a cross-sectional view of casings 20a and 20b containing b. As shown in FIG. 2, the heater-equipped NOx absorbent 1
9a and 19b are formed in such a manner that metal thin plates 26 and metal corrugated plates 27 are alternately and concentrically wound, and NOx is formed by these metal thin plates 26 and metal corrugated plates 27.
An absorbent is loaded. Further, current is caused to flow through the metal thin plate 26 and the metal corrugated plate 27 to cause the metal thin plate 26 and the metal corrugated plate 27 to generate heat, whereby NOx carried by the metal thin plate 26 and the metal corrugated plate 27 is generated.
The absorbent is heated. Therefore, the metal thin plate 26 and the metal corrugated plate 27 constitute a NOx absorbent and also function as a heater.

The first air-fuel ratio sensor 25a and the second air-fuel ratio sensor 25b form an anode on the inner surface of a cylindrical body made of, for example, zirconia, and form a cathode on the outer surface.
Further, it has a structure in which the outside of the cathode is covered with a porous layer, and a current I that varies according to the air-fuel ratio flows between the anode and the cathode of the first air-fuel ratio sensor 25a and the second air-fuel ratio sensor 25b. . This current I is converted into a voltage by the corresponding current-to-voltage conversion circuits 28 and 29 , respectively, and the output terminal of each of the current-to-voltage conversion circuits 28 and 29 is changed according to the air-fuel ratio A / F as shown in FIG. A voltage V is generated. Therefore, the air-fuel ratio, that is, the oxygen concentration in the exhaust gas can be detected from the output voltages V of the current-voltage conversion circuits 28 and 29.

The electronic control unit 30 is composed of a digital computer and is connected to a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, and an input port 35 mutually connected by a bidirectional bus 31. And an output port 36. As shown in FIG. 1, each current-voltage conversion circuit 2
The output voltages of 8, 29 are inputted to the input port 35 via the corresponding AD converters 37, 38. Further, a load sensor 40 that generates an output voltage proportional to the amount of depression of the accelerator pedal 39 is provided, and the output voltage of the load sensor 40 is input to the input port 35 via the AD converter 41. Further, the input port 35 is connected to a rotation speed sensor 42 that generates an output pulse representing the engine rotation speed. on the other hand,
The output port 36 is connected to the fuel injection valve 4, the actuator 18, the control valves 22a, 22
b, the supply pump 23, the heater of the first NOx absorbent 19a, and the heater of the second NOx absorbent 19b.

The NOx absorbents 19a and 19b housed in the casings 20a and 20b use, for example, alumina as a carrier, on which potassium K, sodium N
a, lithium Li, at least one selected from alkali metals such as cesium Cs, alkaline earths such as barium Ba and calcium Ca, rare earths such as lanthanum La and yttrium Y, and noble metals such as platinum Pt. Is carried. Engine intake passage and NOx absorbent 19
The ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the NOx absorbents 19a and 19b is referred to as the air-fuel ratio of the exhaust gas flowing into the NOx absorbents 19a and 19b.
The a and 19b absorb and release NOx when the air-fuel ratio of the inflowing exhaust gas is lean and release the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases. When fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NOx absorbents 19a and 19b, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3. Therefore, in this case, the NOx absorbents 19a and 19b absorb NOx when the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is lean, and the oxygen concentration in the air-fuel mixture supplied into the combustion chamber 3 decreases. NO absorbed when lowered
x will be released. In a diesel engine as shown in FIG. 1, the combustion is normally performed in all operating states with an excess air ratio of 1.0 or more, that is, an average air-fuel ratio of the air-fuel mixture in the combustion chamber 3 is lean. Therefore, the NOx discharged at this time is absorbed by the NOx absorbents 19a and 19b.

If the above-mentioned NOx absorbents 19a and 19b are arranged in the engine exhaust passage, the NOx absorbents 19a and 19b are disposed.
b actually performs the absorption and release of NOx, but there are also parts where the detailed mechanism of the absorption and release is not clear. However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG. Next, regarding this mechanism, platinum Pt and barium Ba are deposited on the carrier.
Will be described as an example in the case of carrying other precious metals,
The same mechanism is obtained by using an alkali metal, an alkaline earth, or a rare earth.

That is, in a diesel engine, a large amount of oxygen is present in the exhaust gas, and the oxygen O ₂ adheres to the surface of the platinum Pt in the form of O ₂ ⁻ as shown in FIG. On the other hand, NO in the inflowing exhaust gas O ₂ on the surface of the platinum Pt ^-
To form NO ₂ (2NO + O ₂ → 2NO ₂ ).
Then part of the produced NO ₂ while bound to the has been barium oxide BaO absorbed into the absorbent while being oxidized on the platinum Pt 4 nitrate ions as shown in (A) NO ₃ ^-
Diffuses into the absorbent in the form of In this way, NOx becomes N
It is absorbed in the Ox absorbents 19a and 19b.

[0014] NO ₂ is produced on the surface of the platinum Pt so long as the oxygen concentration in the inflowing exhaust gas is high, as long as NO ₂ to NOx absorption ability of the absorbent is not saturated it is absorbed in the absorbent and nitrate ions NO ₃ ^- is Generated. On the other hand, when the oxygen concentration in the inflowing exhaust gas decreases and the amount of generated NO ₂ decreases, the reaction proceeds in the opposite direction (NO ₃ ⁻ → NO ₂ ), thus the nitrate ion NO ₃ ⁻ in the absorbent. There are released from the absorbent in the form of NO _2. That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx absorbents 19a and 19b.

In the embodiment shown in FIG.
Oxygen concentration of the NOx absorbent 19
a, reduction to reduce NOx released from 19b
HC is supplied from the agent supply valves 21a and 21b.
You. That is, the hydrocarbon H is supplied from the reducing agent supply valves 21a and 21b.
When C is supplied, the hydrocarbon HC is shown in FIG.
Platinum Pt O_Two ^-Reacts immediately and oxidizes
Can be O on platinum Pt _Two ^-Decreases, the surrounding O_Two
Is O_Two ^-On the platinum Pt in the form of_Two
^-When O adheres, this O_Two ^-Immediately reacts with hydrocarbon HC
Accordingly, the hydrocarbon HC is oxidized. Therefore reducing agent
When hydrocarbon HC is supplied from supply valves 21a and 21b
The oxygen concentration in the exhaust gas drops sharply.

On the other hand, when the oxygen concentration in the exhaust gas decreases and O ₂ ⁻ on the platinum Pt decreases, the reaction proceeds in the direction of (NO ₃ ⁻ → NO ₂ ), thus releasing NO ₂ from the absorbent. Will be done. This NO ₂ reacts with hydrocarbon HC and is reduced. In this way, when NO ₂ is no longer present on the surface of platinum Pt, NO ₂ is successively removed from the absorbent.
₂ is released. Therefore, when hydrocarbon HC is supplied from the reducing agent supply valves 21a and 21b, NOx is released from the NOx absorbents 19a and 19b in a short time, and the released NOx is reduced.

When releasing NOx from the NOx absorbents 19a and 19b, the hydrocarbon HC supplied from the reducing agent supply valves 21a and 21b is converted into the NOx absorbent 19a for a long time.
a, 19b. Therefore, for example, when NOx is to be released from the first NOx absorbent 19a in FIG. 1, the inlet of the first exhaust passage 16a is closed by the flow path switching valve 17 as shown in FIG. However, in this case, when the inlet of the first exhaust passage 16a is completely closed by the flow path switching valve 17 to completely stop the gas flow in the first exhaust passage 16a, the hydrocarbon supplied from the reducing agent supply valve 21a HC is NOx absorbent 1
It takes time to diffuse all over 9a. Thus, in the embodiment shown in FIG. 1, the flow path switching valve 17 slightly reduces the inlet of the first exhaust path 16a so that the exhaust gas slightly flows into the first exhaust path 16a through the flow path switching valve 17 at this time. It is held in the position just opened. Similarly, when NOx is to be released from the second NOx absorbent 19b, the flow path switching valve 17 is held at a position where the inlet of the second exhaust passage 16b is slightly opened. Either NOx absorbent 1
While the NOx releasing action is being performed in 9a and 19b, most of the exhaust gas is discharged from the other NOx absorbent 19a and 1b.
9b, and the other NOx absorbent 19a,
NOx in the exhaust gas is absorbed by 19b.

The NOx absorbents 19a and 19b are N
When the temperature of the Ox absorbents 19a and 19b becomes approximately 200 ° C. or higher, NOx is absorbed if the air-fuel ratio of the exhaust gas becomes lean, and NOx is released if hydrocarbon HC is supplied from the reducing agent supply valves 21a and 21b. . In this case, the temperature of the NOx absorbents 19a and 19b is set to a certain temperature or more, for example, 500 to promote the NOx releasing action at the time of releasing NOx.
It is preferable to raise the temperature to at least ° C. Therefore, in the embodiment shown in FIG. 1, each of the NOx absorbents 19a and 19b is heated by the heater.

However, the inflowing exhaust gas contains sulfur, and the NOx absorbents 19a and 19b absorb not only NOx but also sulfur. This NOx absorbent 19
It is considered that the absorption mechanism of sulfur into a and 19b is the same as the absorption mechanism of NOx. That is, as described in the description of the NOx absorption mechanism, platinum P
If the will be described as an example case of carrying t and barium Ba, the oxygen O ₂ is O ₂ when the air-fuel ratio of the inflowing exhaust gas as described above is lean ^- have been deposited on the surface of the platinum Pt in the form of, influx SO ₂ in the exhaust gas reacts with O ₂ ^{− on} the surface of the platinum Pt to become SO ₃ . Next, the generated SO ₃ is further oxidized on the platinum Pt, is absorbed in the absorbent, and is bonded to barium oxide BaO, and diffuses into the absorbent in the form of sulfate ions SO ₄ ^2- . Next, this sulfate ion SO ₄ ^2- is combined with barium ion Ba ²⁺ to form sulfate BaSO ₄ . In this case, as the amount of sulfate BaSO ₄ increases, the absorption capacity of NOx decreases as the amount of sulfate BaSO ₄ increases. Accordingly, the sulfur oxides SOx absorbed together with NOx by the NOx absorbents 19a and 19b also emit NOx when NOx is released.
Must be released with it.

As described above, when the air-fuel ratio of the inflowing exhaust gas is lean, that is, when the oxygen concentration in the gas flowing around the NOx absorbents 19a and 19b is high, the SOx is reduced together with the NOx.
x is absorbed by the NOx absorbents 19a and 19b. At this time, when the temperature of the NOx absorbents 19a and 19b is relatively low, for example, when the temperature is about 200 ° C. to 350 ° C., the absorbed SOx does not become very stable in the NOx absorbents 19a and 19b. Is supplied with hydrocarbon HC from the reducing agent supply valves 21a and 21b.
Is released from the NOx absorbents 19a and 19b together with NOx. In this case, when SOx and NOx are to be released, if the temperature of the NOx absorbents 19a and 19b is raised to, for example, 500 ° C. or more, the SOx and NOx
Is immediately released from the NOx absorbents 19a and 19b.

However, when the air-fuel ratio of the inflowing exhaust gas is lean, that is, when the oxygen concentration in the gas flowing around the NOx absorbents 19a and 19b is high, the NOx absorbents 19a and 19b
If the temperature of b is raised to, for example, 500 ° C. or more, NO
The SOx absorbed in the x absorbents 19a and 19b is NOx
It is stabilized in the absorbents 19a and 19b, and the sulfate BaSO ₄ generated at this time is not easily decomposed. Therefore, in this case, the hydrocarbon H is supplied from the reducing agent supply valves 21a and 21b.
Even if C is supplied, SOx is removed from the NOx absorbents 19a and 19b.
Will not be released. That is, the NOx absorbent 19
When the temperature of the NOx absorbents 19a and 19b is increased when the oxygen concentration in the gas flowing around the
Can not be released.

By the way, as described above, NOx and SO
In order to promote the release action of x, the NOx absorbent 19a,
It is preferable to raise the temperature of the NOx absorbent 19a, 19b when supplying hydrocarbon HC from the reducing agent supply valves 21a, 21b. However, reducing agent supply valve 2
When the supply of the hydrocarbon HC is started from 1a, 21b, the oxygen concentration in the gas around the NOx absorbents 19a, 19b is still high.
If the temperature of the NOx absorbent 19a is increased,
SOx absorbed in the NOx absorbent 19a and 19b
a, 19b, and this SOx
Can not be released.

Therefore, in the present invention, in order to release NOx and SOx satisfactorily, the hydrocarbon HC is supplied from the reducing agent supply valves 21a and 21b, and then the NOx absorbents 19a and 19b are consumed after the hydrocarbon HC consumes oxygen. Is set to a certain temperature or higher, for example, 500 ° C. or higher. This is the basic NOx emission control according to the present invention. In order to execute the NOx release control, in the embodiment according to the present invention, after the supply of the hydrocarbon HC from the reducing agent supply valves 21a and 21b is started, the air-fuel ratio sensors 25a and 25 are used.
b, it is determined whether or not the oxygen concentration in the exhaust gas that has passed through the NOx absorbents 19a and 19b has become substantially zero, and the oxygen concentration in the exhaust gas that has passed through the NOx absorbents 19a and 19b has become substantially zero. NOx absorbent 19
The heating action of the heater carrying the a and b is started. However, the NOx absorbent 19a, 1
When the oxygen concentration in the exhaust gas passing through the exhaust gas 9b becomes substantially zero, the temperature of the NOx absorbents 19a and 19b becomes constant,
For example, the NOx absorbent 19a,
The heating operation of the heater carrying the NOx absorbents 19a and 19b can be started before the oxygen concentration in the exhaust gas passing through 9b becomes substantially zero.

Next, an embodiment of the NOx release control will be described with reference to FIG. Referring to FIG. 5, it can be seen that the switching operation of the flow path switching valve 17 causes the inlet of the first exhaust passage 16a and the inlet of the second exhaust passage 16b to be alternately closed. When the inlet of the first exhaust passage 16a is closed by the flow switching valve 17, the first control valve 22a is opened, and the supply of the hydrocarbon HC from the first reducing agent supply valve 21a is started. . The operation of supplying the hydrocarbon HC is continued for a predetermined period of time. Hydrocarbon H
When the supply of C is started, the oxygen concentration in the exhaust gas that has passed through the first NOx absorbent 19a gradually decreases, and thus the output of the first air-fuel ratio sensor 25a, actually, the output of the current-voltage conversion circuit 28 The voltage V gradually decreases. Next, when the output voltage V of the current-voltage conversion circuit 28 becomes substantially zero,
When the oxygen concentration in the exhaust gas passing through the NOx absorbent 19a becomes substantially zero, the first heater carrying the first NOx absorbent 19a is energized for a certain time. After the power supply to the first heater is stopped, the temperature of the first NOx absorbent 19a is reduced, and then the inlet of the second exhaust passage 16b is closed by the flow path switching valve 17. Since the exhaust gas temperature of a diesel engine is low, the first N
The temperature of the Ox absorbent 19a drops immediately. Therefore the first
When the power supply to the heater is stopped, the flow path switching valve 17 is driven immediately to open the inlet of the first exhaust passage 16a,
The inlet of the second exhaust passage 16b may be closed by the flow path switching valve 17.

When the inlet of the second exhaust passage 16b is closed by the flow path switching valve 17, the second control valve 22b is opened, so that the supply of the hydrocarbon HC from the second reducing agent supply valve 21b. Be started. The operation of supplying the hydrocarbon HC is continued for a predetermined period of time. When the supply of the hydrocarbon HC is started, the oxygen concentration in the exhaust gas that has passed through the second NOx absorbent 19b gradually decreases, and thus the output of the second air-fuel ratio sensor 25b, actually, the current-voltage conversion circuit 2
9 gradually decreases. Next, when the output voltage V of the current-voltage conversion circuit 29 becomes substantially zero, that is, the second NO
When the oxygen concentration in the exhaust gas passing through the x-absorber 19b becomes substantially zero, the second NOx absorbent 19b
The heater is energized for a certain time. After the energization of the second heater is stopped, the inlet of the first exhaust passage 16a is closed again by the passage switching valve 17 after the temperature of the second NOx absorbent 19b decreases. In this case as well, when the power supply to the second heater is stopped, the flow path switching valve 1 is immediately turned on.
7, the inlet of the second exhaust passage 16b may be opened, and the inlet of the first exhaust passage 16a may be closed by the flow path switching valve 17.

As described above, the reducing agent supply valves 21a, 21a
When the hydrocarbon HC is to be supplied from b, the corresponding control valves 22a and 22b are opened for a certain time. However, the amount of NOx absorbed by the NOx absorbents 19a and 19b per unit time increases as the engine speed N increases, and increases as the engine load increases. Therefore, the hydrocarbon HC supplied from the reducing agent supply valves 21a and 21b
6A is increased as the engine speed N is increased as shown in FIG. 6A, and is increased as the depression amount L of the accelerator pedal 39 is increased as shown in FIG. 6B. . In the embodiment shown in FIG. 1, the supply amount of hydrocarbon HC is controlled by controlling the drive voltage E of the supply pump 23, and FIGS. 6A and 6B
The drive voltage E required to obtain the supply amount of hydrocarbon HC shown in FIG. 6 is stored in the ROM 32 in advance in the form of a map shown in FIG. 6C as a function of the engine speed N and the depression amount L of the accelerator pedal 29. Have been.

FIG. 7 shows a NOx release control routine, which is executed by interruption every predetermined time. Referring to FIG. 7, first, at step 50, it is determined whether or not a flag F indicating that NOx should be released from the first NOx absorbent 19a is set. When the flag F is set, the routine proceeds to step 51, where the passage switching valve 17 is controlled by the actuator 18 so as to close the inlet of the first exhaust passage 16a and open the inlet of the second exhaust passage 16b. You.
Next, at step 52, the drive voltage E of the supply pump 23 is calculated from the map shown in FIG. Next, at step 53, the supply pump 23 is driven according to the drive voltage E. Next, at step 54, a process of opening the first control valve 22a for a certain period of time is performed, and thus the supply of the hydrocarbon HC from the first reducing agent supply valve 21a is started.

Next, at step 55, it is determined whether or not the output voltage V of the current-voltage conversion circuit 28 of the first air-fuel ratio sensor 25a has become substantially zero, that is, the oxygen concentration in the exhaust gas passing through the first NOx absorbent 19a has become substantially zero. Is determined. Process for energizing predetermined time a first heater carrying the output voltage V Gaho crucible proceeds to step 56 becomes zero the 1NOx absorber 19a of the voltage-current conversion circuit 28 is performed. Next, at step 57, the flag F is reset.

Next, when an interrupt is performed, step 5 is executed.
When it is determined that the flag F is not set at 0, the routine proceeds to step 58. In step 58, the actuator 18 controls the switching of the flow path switching valve 17 so as to open the inlet of the first exhaust passage 16a and close the inlet of the second exhaust passage 16b. Next, at step 59, the drive voltage E of the supply pump 23 is calculated from the map shown in FIG. Next, at step 60, the supply pump 23 is driven according to the drive voltage E. Next, at step 61, a process of opening the second control valve 22b for a certain period of time is performed, and thus the supply of the hydrocarbon HC from the second reducing agent supply valve 21b is started.

Next, at step 62, it is determined whether or not the output voltage V of the current-voltage conversion circuit 29 of the second air-fuel ratio sensor 25b has become substantially zero, that is, the oxygen concentration in the exhaust gas that has passed through the second NOx absorbent 19b has become substantially zero. Is determined. When the output voltage V of the voltage-to-current conversion circuit 29 becomes almost zero, the routine proceeds to step 63, in which a process for energizing the second heater carrying the second NOx absorbent 19b for a predetermined time is performed. Next, at step 64, the flag F is set.

Although the case where the present invention is applied to a diesel engine has been described above, it is needless to say that the present invention can be applied to a gasoline engine.

[0032]

According to the present invention, NOx and SOx can be quickly released from the NOx absorbent at the time of NOx release while preventing the sulfur oxide SOx from remaining stored in the NOx absorbent.

[Brief description of the drawings]

FIG. 1 is an overall view of an internal combustion engine.

FIG. 2 is a cross-sectional view of a casing containing a heater-equipped NOx absorbent.

FIG. 3 is a diagram illustrating an output voltage of a current-voltage conversion circuit.

FIG. 4 is a diagram for explaining an absorption / release effect of NOx.

FIG. 5 is a time chart of NOx release control.

FIG. 6 is a diagram for explaining a supply amount of a hydrocarbon.

FIG. 7 is a flowchart for performing NOx release control.

[Explanation of symbols]

 16a, 16b ... exhaust passage 19a, 19b ... NOx absorbent 21a, 21b ... reducing agent supply valve 25a, 25b ... air-fuel ratio sensor

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ Identification code FI F01N 3/24 (56) References JP-A-3-135417 (JP, A) JP-A-4-76923 (JP, A) JP-A-63-150441 (JP, A) JP-A-49-43870 (JP, A)

Claims (1)

(57) [Claims] An NOx absorbent that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas is reduced, is placed in the engine exhaust passage. And a heating means for heating the NOx absorbent.
When the concentration of oxygen flowing into the NOx absorbent is decreased in order to release oxygen, the oxygen concentration in the exhaust gas coming into contact with the NOx absorbent falls below a predetermined concentration, and then the temperature is increased by the heating action of the heating means. An exhaust gas purifying apparatus for an internal combustion engine, wherein a heating operation of the heating means is controlled so that the temperature of the NOx absorbent exceeds a predetermined temperature.